Doctors have treated only three leukemia patients, but the sensational results from a single shot could be one of the most significant advances in cancer research in decades. And it almost never happened. In the research published Wednesday, doctors at the University of Pennsylvania say the treatment made the most common type of leukemia completely disappear in two of the patients and reduced it by 70 percent in the third. In each of the patients as much as five pounds of cancerous tissue completely melted away in a few weeks, and a year later it is still gone